This invention relates to computer memory in general, and more specifically to sensing data that is stored in non-volatile memory.
Non-volatile semiconductor memory is memory that can retain information when a power supply is switched off. Non-volatile memory, such as flash memory, is being used increasingly in electronic devices for personal and commercial use, including cellular telephones, digital cameras, embedded devices, and personal data assistants. Technology has made it possible to produce flash memory that is increasingly dense, resulting in greater and greater amounts of memory being available to electronic products. In order to reduce the power consumption of these products, there has also been an attempt to operate flash memory at lower voltages. The combination of greater memory density, demands for better read performance at increasing speed, and lower supply voltages has created design challenges.
In addition, the operation of non-volatile memory has changed. Rather than operating such that read functions and write functions do not occur at the same time, non-volatile memory is developing into read-while-write operations, in which some memory cells may be written to at the same time that other memory cells are read. The need to provide flexibility to support read-while-write operations has introduced further challenges in the design of non-volatile memory.
Flash memory is composed of memory cells that are typically arranged in an array of rows and columns. The memory cells then may be combined to form memory blocks or other larger units of memory cells. In utilizing such flash memory, sensing circuits are provided to determine what logical value is stored within each of the memory cells. The data signal each memory cell produces is small and thus a sensing amplifier is used to determine the cell contents. Typically, a single set of sensing amplifier circuits, which may be termed xe2x80x9cglobal sensing circuitsxe2x80x9d, is used for a flash memory array, as is shown in FIG. 1. In this example, a sense node connects the memory cells to the global sensing circuits. The use of global sensing circuits requires that the sense node remain active at any time the contents of a flash memory cell are read. Because the sense node is in constant operation and extends physically across the memory cell array, the sense node has significant capacitance. The charging of this node causes a reduction in read speed, an increase in power consumption, and a possible need for an increase in device sizes to improve the read performance.
As the size of a non-volatile memory array is increased, the sense node connecting the memory cells to the sensing amplifier circuits will increase in length, thereby increasing the parasitic capacitance and power consumption of the non-volatile memory device. These factors limit the speed and size of non-volatile memory devices.
FIG. 1 illustrates the structure of a conventional non-volatile memory device. FIG. 1 does not show all aspects of a non-volatile memory device, but instead is limited to the features that are most relevant to the sensing of non-volatile memory content. Within the non-volatile memory device 100, there is a plurality of memory cells 110. In this illustration, the array of memory cells 110 is divided into a first array of memory cells 120 and a second array of memory cells 130. Between first array 120 and second array 130, there are multiplexers (or Y-decoders) 150, which choose the signals that are subject to the sensing operation. Connected to multiplexers 150 is sense node 140. Sense node 140 traces through the memory device 100 to a plurality of global sensing circuits 160, which serve to detect and amplify the signals from the appropriate memory cells. The output of global sensing circuits 160 then becomes an output 170 of memory device 100. In this illustration, the signal representing the stored content of any memory cell within memory device 100 will be conducted by sense node 140, and thus sense node 140 remains active for any read operation.